This proposal specifically aims: 1) to further develop tissue culture techniques for isolated adult liver parenchymal cells in order to maximize the rate of DNA synthesis, 2) to study the content, labeling pattern and turnover of the cyclic nucleotides, particularly, cyclic adenosine monophosphate (cAMP) during growth stiumlation of hepatocytes in vitro, 3) to investigate the role which diadenosine tetraphosphate (AP4A) may play as a signal to the onset of growth, 4) to carry out a comparison of nucleoside tri-, di- and monophosphates as well as ATP/ADP ratios in quiescent and growth stiumlated cells, 5) to continue to scrutinize cellular extracts for the presence of "novel" (hitherto unidentified) but proliferation specific nucleotides, and 6) to seek supportive evidence for in vitro findings in whole animal experiments, where feasible. Some recently developed methodology high rates of DNA synthesis in hepatocytes in culture by various combinations of hormones, nutrients and growth factors should considerably enhance the search for nucleotide changes and patterns which correlate with growth. The techniques of high pressure liquid chromatography (HPLC), radioimmunoassy (RIA), and labeling with radioactive precursors will be applied to the analysis and rates of turnover of the nucleotide pools (cyclic nucleotides, AP4A and nucleoside tri-, di- and monophosphates). Adult hepatocytes, normally quiescent but endowed with impressive latent growth capacity, provide a model for study of growth regulation in body tissues. An understanding of the mechanisms involved is relevant not only in the specific context of liver regeneration, but in the general context of repair and replacement of lost or damaged tissues elsewhere in the body, and in management of neoplasia as well.